The "kinds" of animals selected had reference to the clear-cut and unalterable boundaries or limits set by the Creator, within which boundaries creatures are capable of breeding "according to their kinds." It has been estimated by some that the hundreds of thousands of species of animals today could be reduced to a comparatively few family "kinds"the horse kind and the cow kind, to mention but two. The breeding boundaries according to "kind" established by Jehovah were not and could not be crossed
These estimates may seem too restrictive to some, especially since such sources as The Encyclopedia Americana indicate that there are upwards of 1,300,000 species of animals. (1977, Vol. 1, pp. 859-873) However, over 60 percent of these are insects. Breaking these figures down further, of the 24,000 amphibians, reptiles, birds, and mammals, 10,000 are birds, 9,000 are reptiles and amphibians, many of which could have survived outside the ark, and only 5,000 are mammals, including whales and porpoises, which would have also remained outside the ark. Other researchers estimate that there are only about 290 species of land mammals larger than sheep and about 1,360 smaller than rats. (The Deluge Story in Stone, by B. C. Nelson, 1949, p. 156; The Flood in the Light of the Bible, Geology, and Archaeology, by A. M. Rehwinkel, 1957, p. 69) So, even if estimates are based on these expanded figures, the ark could easily have accommodated a pair of all these animals.